Apparatus for visually inspecting strip travelling at high speed



Nov. 19, 1963 G. DYKEMAN ETAL 3,111,555

APPARATUS FOR VISUALLY INSPECTING STRIP TRAVELLING AT HIGH SPEED 2 Sheets-Sheet 1 Filed May 16, 1960 INVENTORS GEORGE DY/(E MAN and JAMES A. MIL/V5.5 y 44, 9

% Attorney Filed May 16, 1960 2 Sheets-Sheet 2 Nov. 19, 1963 G. DYKEMAN ETAL 3,111,555

APPARATUS FOR VISUALLY INSPECTING STRIP TRAVELLING AT HIGH SPEED 7 I y- A J Generator Pulse Type Tact; Generator Flost; Lamp Po wer Supply 22 and Tr/gger Camera 39 40 Con trol .x 47 I Flip Flop I Syn. 48 Generator 38 Preset Display Counter T/mer 44 7 45 4/ IN VEN TORS A uto. Mqnug/ GEORGE 0 Y/(E MAN and 43 JAMES A. MIL/V5.5

Attorney United States Patent 3,111,555 APPARATUS Fflll VlSUALLY INSPECTWG STRIP TRAJELLENG All SPEED George Bykeman, Pittsbur h, and James A. Miines,

Pitcairn, Pa., assignors to United States Steel Corporation, a corporation of New .lerse Filed May 16, 196%, Ser. No. 29,364 4 Claims. (Cl. 1786.8)

This invention relates to an apparatus for visually inspecting the surface of a rapidly moving object. It relates more particularly to an apparatus for spot-checking the surface condition of metal strip, such as tin plate, as it travels through a processing line.

In metal strip processing lines, quality control of the material is determined by inspectors stationed at fixed positions along the length of the line who visually inspect the material for defects therein. V-fhen sheets are being produced, visual inspection of strip material such as tin plate can be performed satisfactorily at line speeds of up to 750 feet per minute by withdrawing sheets from the line for closer inspection. However, recent operating procedures call for the production of tin plate in coil form rather than sheets, and for operating line speeds in excess of 1,000 feet per minute. These procedures prevent quality inspection of the strip material by withdrawing samples from the line and have presented a problem from the standpoint of classifying the material on the basis of surface blemishes therein.

In the production of tin plate, irregularities in rolling, cleaning, plating, or melting operations cause surface lemishes that may impair the desired mirror like finish. These blemishes, or defective areas, may be large, covering almost the entire width of the strip, or small, in the order of 1 inch diameter spots. Classification of the product into prime, seconds, and waste depends upon an evaluation of the type and nature of the defect, as well as its size and distribution.

When tin plate is produced in sheet form, quality evaluation is made on the basis of direct visual inspection of sheets in motion or sheets withdrawn from piles. However, when tin plate is produced in coil form, quality evaluation cannot be made on the basis of visual inspection, not only because line speed is increased in coil production, but also because the opportunity to spot-check sheets is eliminated. One of the principal objects of this invention is, accordingly, to provide an apparatus for scanning the surface of tin plate strip for surface blemishes as it travels through a tinning or other processing line.

A further object of the invention is to provide an apparatus that will optically stop the movement of strip travelling through a processing line and produce an image of a portion of the surface area thereof that can be retained for a period of time suflicient to enable visual observation of its surface condition.

Another and more specific object of the invention is to provide an apparatus for visually observing the surface condition of strip travelling at a high rate of speed that comprises a flash lamp that is triggered to momentarily illuminate a portion of the strip surface for a short period of time in the nature of several microseconds and thus in effect optically stop the movement of the strip, and an industrial television camera of the image storage type for viewing the portion of the strip surface while it is illuminated by the flash lamp. In a manner to be described, the television camera includes an electronic screen on which an image of the illuminated strip surface portion in the form of an electrical charge pattern is produced and retained for a period of time to enable a visual observation thereof. A television receiver is provided for displaying the image on the camera screen for visual observation.

Other objects and advantages of the invention of the invention will become apparent from the following description.

In the drawings, there is shown a preferred embodiment of the invention. In this showing:

FIGURE 1 is a diagrammatic View in side elevation illustrating a preferred arrangement of the essential components of the apparatus of this invention with respect to strip travelling through a processing line;

FIGURE 2 is a plan view of a portion of the apparatus shown in FIGURE 1;

FIGURE 3 is an end View looking from the right of FIGURE 2;

FIGURE 4 is a view similar to FIGURE 3 illustrating a modification that eliminates the light diffusing panel in the modification shown in FZGURES 1, 2, and 3;

FIGURE 5 is a schematic sketch of a television apparatus that uses a vidicon as the camera tube for viewing the surface of strip as it travels through a processing line;

FIGURE 6 is a fragmentary sectional view of the target element of the camera tube shown in FIGURE 5; and

FIGURE 7 is a diagrammatic and schematic illustration of an apparatus for controlling the flashing operation of the strip illuminating lamps shown in the preceding figures.

The apparatus of this invention provides for visual inspection of the surface of a rapidly moving object such as metal strip 1 as it is moved through a processing line, for example, by drive bridles 2 and 3 to a recoiler 4. Such apparatus, generally stated, comprises a flash lamp system that includes a pair of lamps 5 disposed on opposite sides of the strip 1, which are operated to momentarily illuminate the surface of the strip for a short period of time in the nature of a few microseconds and thus optically stop its movement, a television camera 6 of the image-storage type for viewing the illuminated surface of the strip, and a television receiver 7 for visually displaying the image of the surface that is picked up by the camera 6. When the apparatus is used to observe the surface condition of material having a highly reflective or mirror-like surface, such as tin plate, the lamps 5 are directed upwardly against a light diffuser plate 3, which is arranged in a horizontal position above and parallel to the surface of the strip to be viewed and operates in a manner to be described to illuminate the area of the strip to be viewed by specularly diffused light. The camera 6 is mounted as shown in FIGURES 1-3 and 7 to view such area through a small opening 9 in the diffuser plate 8. When the material to be inspected has a surface that is less reflective, such as steel billets or black plate, the diffuser plate 8 is not necessary and the flash lamps 5 are directed against the surface of the strip as shown in FIG- URES 4 and 5 of the drawings.

Defects in tin plate fall basically into two optical groups: those defects which, representing damage to the surface, have different total reflection or different light absorption; and other defects which have the same total reflection as the rest of the surface but differ in specular or diflused reflection. Defects in the first group can be photographed with light which is totally reflected into a camera lens and result in photographic prints showing the perfect surface white, and the defect gray to black. Defects in the second group cannot be delineated in this manner because their total reflection is the same as the perfect surface and photographic prints thus register white on a white background. However, both types of defects have a common characteristic in that they differ from the perfect surface in the amount of specular or diffused reflection, and the diffuser plate 8 shown in FIGURES 1-3 is effective to produce an optical image by specular reflection only that is applicable to both optical groups of tin plate defects. For this purpose, the diffuser plate 8 may be a sheet of plywood or metal that has its under surface 1%) painted white and is provided with a nonreflective stop or flag 11, preferably, in the form of a rectangular sheet of black felt covering a portion of the light diffusing surface it). When the plate 8 is illuminated by light from the larnps 5, diffused light will be reflected from the white surface or border it) about the nonreflective stop or flag 11 against the upper surface of the strip 1, and the flag 11 will appear as a dark image on the upper surface of the strip 1. The camera 6 is mounted so that it views this image through the opening 9. Since perfect tin plate acts as a first-surface mirror, the camera 6 will see only a black image as long as the tin plate surface is perfect. However, since all defects modify the specular or diffused reflection characteristic of the perfect surface, such defects will reflect some light to the camera 6 and will be observed as a bright spot on a black background. Since the black felt block 11 absorbs all light rays that would otherwise be reflected onto the tin plate and subsequently from the mirror-like finish of the tin plate into the viewing camera 6, the camera 6 will see only a relatively dark background (reflection of the optical stop iii) if perfect tin plate passes through the inspection area. However, if flaws or other discontinuities are contained in the section of tin plate being viewed, some specularly diffused light will be reflected to the camera 6.

In order to optically stop the motion of strip 1 travelling at high linear speeds, the lamps are operated in a manner to be described to produce a very short, bright pulse or flash of light. Since the resulting image that is reflected from the surface of the strip to the camera 6 exists only for a very short period of time, in the nature of a few microseconds, during which the surface of the strip is illuminated, the camera 6 as indicated must be one of the image-storage type that will retain the image a longer period of time, several seconds for example, to enable its being visually observed on the television receiver. There are several forms of conventional television tubes that will store images for varying times and are suitable for the purposes of this invention. Generally stated, image-storage tubes of this character may be characterized as having a target screen on which an electrical charge pattern corresponding to the light intensity of the image viewed is produced and retained for a period of at least several scanning frames to enable its being read out and visually displayed on a television receiver. Such tubes may be those in which a charge pattern is produced directly by photo-conduction or photo-emissivity, and the resulting image may be read out on charge restoration principles or charge modulation principles. Examples of these light-operated storage tubes are the Emitron, Storage Orthicon, most Vidicons, but in particular the RCA 6198 Vidicon, and the Westinghouse Permachon.

FIGURES 5 and 6 illustrate an apparatus similar to that shown in FIGURE 4 in which the camera 6 is provided with a vidicon-type television tube 14 which has a photoconductive or light responsive target or screen 15 that is inherently an image-storage type. In this arrangement, an image of the surface 16 of the strip 1, produced by flashing operation of the lamps 5, is focused by a lens 17 on the target 15 of the tube 14. The tube target 15 as will be apparent from the fragmentary showing of FIGURE 6 comprises the glass face plate 18 of the tube 14 which has a transparent conducting signal plate 19 on its outer face, and on its inner face a mosaic of a large number of mutually insulated photoconductive elements Ell that are capacitively coupled with the common signal plate 19. An electronic beam 21 originating in an electron gun or camera control 22 scans the target 15 and operates, in the absence of light, to place the mosaic elements 26 at cathode potential. When an image is focused on the target 15, the elements 20 become charged positively according to the intensity of the illumination of the image focused on the transparent signal plate 19. The circuit for producing and effecting scanning operation of the beam 21 comprises the camera control 2 2 and a synchronizing control and deflection generator 23, which include battery 24, load resistor 25, bias resistors 26 and 27, control grids 28 and 29, cathode 3G, horizontal deflection coil 31, and vertical deflection coil 32.. The load r sistor 25 is connected in series with the signal plate 19, which is capacitively coupled through the dielectric of the face plate 18 and the capacitor terminal elements 20 with the electron beam 21, and light passing through the plate 18 operates to slightly reduce the charge on the target 15 and to generate a signal voltage across load resistor 25 in proportion to the magnitude of the stored electrical charges on the capacitor terminal elements 20. The stored image may thus be repeatedly scanned until the mosaic is brought back to cathode potential. The changes in voltage across the load resistor 25 are passed through a coupling capacitor 33 and amplified and processed in a video amplifier 34. The amplified signals are transmitted to the remote television receiver 7 or monitor where the image on the target 15 is recreated on the face of its cathode ray tube, under the control of deflection generator and synchronizing control 23. As indicated above, the tube 14 retains the image formed on its target screen 14 as the result of momentary illumination of the strip 1 by the flash lamps 5 for a period of time sufficient to enable visual observation of such image on the television monitor 7.

As indicated above, the lamps 5 are operated to produce a very short bright pulse or flash of light and to thereby optically stop the motion of the strip 1. Assun ing that each light pulse has a duration of 30 microseconds, strip travelling at a speed of several hundred feet per minute will travel less than one hundredth of an inch during the period in which it is illuminated, and the motion of the strip will thus be effectively stopped during the formation of the television image on a camera tube screen such as that provided by the target 15 of the Vidicon 14 shown in FIGURE 5.

A preferred system for controlling and synchronizing the flashing operation of the lamps '5 with the scanning operation of the television camera 6 and television monitor 7 is shown in FIGURE 7. Since each of the components used in this system are conventional and a number of different forms of each are commercially available, they have been designated diagrammatically by block diagrams in this showing. This system comprises a flash lamp power supply and trigger circuit unit 37 that triggers each pulse or flashing operation of the lamps 5. (The unit 37 is controlled by the deflection generator and synchronizing control 23 in such manner that it is conditioned for effecting a flashing operation of the lamps 5 only during the aly-back intervals between successive scann-ing frames of the camera 6 and receiver 7, which are controlled by the synchronizing generator 23 as indicated diagrammatically in FIGURE 7. For this purpose, hig gering operation of the unit 37 is under the control of a display timer 38 that operates through a flip-flop control circuit unit 39 and an amplifier 40 to generate and transmit a flash-lamp trigger impulse to the triggering unit 37, and the flip-flop circuit 39 is connected with the synchronizing generator 23 in such manner that it can operate to send the flash-lamp triggering impulse to the unit 37 only between successive scanning frames of the television camera 6 and receiver monitor 7.

Operation of the display timer 68 is controlled by a switch 41 that includes contacts 42 and 43 which respectively provide for its operation automatically or manually. When the switch 41 is in engagement with the contact 43 a manual push button 44 may be operated to effect single flashing operations of the lamps 5 as dm-ired. When the switch 41 is engaged with automatic contact 42, operation of the display timer 38 is placed under the control of a preset counter 45 that operates to count the pulses fed to it from control circuits 46 and 47 that are selectively connected to the counter 45 by a manual control switch 48. When the counter 45 receives the number of pulses required by its presetting, it operates to send a flash triggering pulse to the display timer 38. When the counter 45 is connected through the switch 48 with the control circuit 46 it counts the pulses, corresponding to scanning frames of the television camera 6 and receiver 35, that are fed to it by the synchronizing generator 23, and after the preselected number of impulses have been counted, a flash triggering pulse is fed to the display timer 38 for effecting a flashing operation of the lamps 5, which are thus operated at predetermined time intervals according to the setting of the counter 45. When the preset counter 45 is connected with the circuit 47, it counts the pulses fed to it by a pulse-type tachometer generator 49 that is connected for rotation by a conveyor or other roll 50 over which the strip 1 travels during its movement through a processing line. Since the rotational movement of the roll 50 corresponds to the linear movement of the strip 1, the pulses generated by the tachometer generator 49 furnish a measure of the length of the strip 1 travelling over the roll 59, and are transmitted by the circuit 47 to the counter 45 which will effect a flashing operation of the lamps 5 at spaced points along the length of the strip 1 that may be preselected by adjusting the setting of the counter '45. From the foregoing, it will be apparent that the system shown in FIGURE 7 provides for selective flashing operation of the lamps 5 either manually, at periodically timed intervals, or at spaced intervals at preselected points along the length of the strip.

While one embodiment of our invention has been shown and described it will be apparent that other adaptations and modifications may be made without departing from the scope of the following claims.

We claim:

1. A visual inspection apparatus of the character described comprising, the combination with a metal strip processing line through which strip travels continuously at a rapid rate, of an apparatus for optically stopping the movement of said strip and obtaining a stopped image of a portion of its surface comprising, a flash lamp, means for triggering the operation of said lamp to momentarily illuminate said strip surface portion, a television camera having an electronic screen on which an image of said illuminated surface portion in the form of an electrical charge pattern is impressed and retained for a period of time greater than the duration of the momentary illumination of said strip, and a television receiver for visually displaying the said electrical charge pattern image on said camera screen, said period of time being sufliciently long to enable visual observation of said electrical charge pattern on said television receiver.

2. A visual inspection apparatus of the character described comprising, the combination with a metal strip processing line through which strip travels continuously at a rapid rate, of an apparatus for optically stopping the movement of said strip and obtaining a stopped image of a portion of its surface comprising, a flash lamp, means for triggering the operation of said lamp to momentarily illuminate said strip surface portion, a television camera having an electronic screen on which an image of said illuminated surface portion in the form of an electrical charge pattern is impressed and retained, a television receiver for displaying the said electrical charge pattern image on said camera screen, and means including a synohronizing generator controlling the scanning operations of said television camera and receiver for operating said triggering means during the fly-back between scanning frames of said receiver and camera.

3. A surface inspection apparatus as defined in claim 2 characterized by said trigger operating means including selectively operable plural means providing respectively for its operation manually, at periodicallly timed intervals, and at spaced intervals along the length of the strip moving through said processing line.

4. A visual inspection apparatus of the character described comprising, the combination with a metal strip processing line through which strip travels continuously at a rapid rate, of an apparatus for optically stopping the movement of said strip and obtaining a stopped image of a portion of its surface comprising, a television camera for viewing portion of said strip surface, a flash lamp for directing a momentary pulse of light against said strip surface portion, said camera having an electronic screen on which an image of said strip surface portion in the form of an electrical charge pattern is impressed and retained for a period of time sufiicient to enable its visual observation, and a television receiver for visually displaying the image on said camera screen.

References Cited in the file of this patent UNITED STATES PATENTS 2,648,723 Goldsmith Aug. 11, 1953 2,798,605 Richards July 9, 1957 FOREIGN PATENTS 803,843 Great Britain Nov. 5, 19,58 

1. A VISUAL INSPECTION APPARATUS OF THE CHARACTER DESCRIBED COMPRISING, THE COMBINATION WITH A METAL STRIP PROCESSING LINE THROUGH WHICH STRIP TRAVELS CONTINUOUSLY AT A RAPID RATE, OF AN APPARATUS FOR OPTICALLY STOPPING THE MOVEMENT OF SAID STRIP AND OBTAINING A STOPPED IMAGE OF A PORTION OF ITS SURFACE COMPRISING, A FLASH LAMP, MEANS FOR TRIGGERING THE OPERATION OF SAID LAMP TO MOMENTARILY ILLUMINATE SAID STRIP SURFACE PORTION, A TELEVISION CAMERA HAVING AN ELECTRONIC SCREEN ON WHICH AN IMAGE OF SAID ILLUMINATED SURFACE PORTION IN THE FORM OF AN ELECTRICAL CHARGE PATTERN IS IMPRESSED AND RETAINED FOR A PERIOD OF TIME GREATER THAN THE DURATION OF THE MOMENTARY ILLUMINATION OF SAID STRIP, AND A TELEVISION RECEIVER FOR VISUALLY DISPLAYING THE SAID ELECTRICAL CHARGE PATTERN IMAGE ON SAID CAMERA SCREEN, SAID PERIOD OF TIME BEING SUFFICIENTLY LONG TO ENABLE VISUAL OBSERVATION OF SAID ELECTRICAL CHARGE PATTERN ON SAID TELEVISION RECEIVER. 